Silicon carbide (SiC) is a desired material for use in the manufacture of semiconductor devices. Silicon carbide has a variety of properties useful in such devices, including a wide bandgap, a high thermal coefficient, a low dielectric constant and high temperature stability. As a result, silicon carbide materials can provide excellent semiconducting properties, and electronic devices made from silicon carbide can perform at higher temperatures as compared to devices made from other materials currently used in the industry.
Traditionally, two broad categories of techniques have been used for forming crystalline silicon carbide for semiconductor applications. The first of these techniques is known as chemical vapor deposition (“CVD”) in which reactant gases are introduced into a system to form silicon carbide crystals, typically in epitaxial layers, upon an appropriate substrate.
The second main technique is generally referred to as sublimation. In this technique, a silicon carbide source material (typically a powder) is used as a starting material. The silicon carbide starting material is heated in a crucible until it sublimes or vaporizes, and the vaporized material is encouraged to condense to produce the desired crystals. This can be accomplished by introducing a silicon carbide seed crystal into the crucible and heating it to a temperature less than the temperature at which silicon carbide sublimes. A pioneering patent that describes methods for forming crystalline silicon carbide for semiconductor applications using such seeded sublimation techniques is U.S. Pat. No. 4,866,005 to Davis et al., issued Sep. 12, 1989, which was reissued as U.S. Pat. No. Re. 34,861, issued Feb. 14, 1995, which patents are incorporated herein by reference as if set forth in their entirety.
Many semiconductor applications require a single crystal material with very low levels of defects in the crystal lattice and/or low levels of unwanted impurities. Even in a pure material, a defective lattice structure can prevent the material from being useful for electrical devices, and the impurities in any such crystal are preferably controlled to give certain desired electrical characteristics (such as an n or p character). For example, crystal imperfections in the SiC single crystal, including micropipe defects, screw dislocations, edge dislocations, stacking faults, and the like, can result in current leakage and reduced breakdown voltage in a SiC device.
One approach to improve SiC quality focuses on seed crystal orientation within the crucible. SiC single crystals include a {0001} plane (also referred to in the art as the c-plane) as the main plane orientation. SiC single crystals also include a {1-100} plane and a {11-20} plane (the a-planes), which are perpendicular to the {0001} plane. Conventionally, sublimation processes such as those described above use the c-plane {0001} of a silicon carbide seed crystal as the growth surface to grow bulk silicon carbide single crystals in the <0001> growth direction.
SiC single crystals grown on a c-plane, i.e., grown in a <0001> direction using a {0001} plane as a seed crystal surface, include micropipe defects, typically at a density up to 103 cm−2. Such SiC crystals can have other defects as well, such as screw dislocations, typically at a density of 103 to 104 cm−2, and edge dislocations, typically at a density of 104 to 105 cm−2, in a direction substantially parallel to the <0001> direction. The defects and dislocations are carried over into devices produced using the SiC single crystal, for example, devices fabricated in an epitaxial layer which is grown onto the SiC crystal. Accordingly, the defects and dislocations will also exist in the epitaxial layer at substantially the same density as in the SiC single crystal wafer and will undesirably affect device characteristics as well.
Growing bulk SiC single crystals on a seed crystal with the a-plane as the growth surface can reduce micropipe and dislocation concentrations in the {0001} plane. The resultant crystal, however, can include other undesired defects, primarily stacking faults located on {0001} planes at a density as high as 102 to 104 cm−1 in a direction substantially parallel to the direction of the crystal growth. Such defects are also carried over into downstream products incorporating the SiC crystal as a component. SiC power devices manufactured from such a SiC wafer can in turn have a relatively high on resistance and a relatively large current leakage in the reverse direction, thereby undesirably affecting performance of the device.
U.S. Patent Application Publication 2003/007611 is directed to a process of manufacturing SiC single crystals using a series of seed crystals having different growth planes. In the '611 application, a first SiC crystal grows in a <1-100> or <11-20> direction (or in an “a-plane” direction) on a first seed crystal having a first growth surface, typically a plane having an inclination of 20 degrees or smaller from a {1-100} or {11-20} plane. The resultant silicon carbide crystal is processed (wafered) to make a second seed crystal having a second growth surface with an inclination of 45 to 90 degrees from a {0001} plane of the first grown crystal. Subsequently, another silicon carbide crystal is grown on the second seed crystal, from which is formed a final seed crystal having a growth surface with a plane having an inclination of 20 degrees or less from a {0001} plane of the second grown crystal.
The '611 application reports that the process reduces edge dislocations and stacking faults. The process, however, suffers various drawbacks. The SiC crystals produced in each sequential step must be processed, i.e., the silicon carbide boule must be wafered, or sliced, polished and ground (to remove polycrystalline growth), to form a seed crystal suitable for use in the next step of the process. Each such step can be time consuming and labor intensive. In addition, the requisite slicing of each subsequently grown SiC crystal boule can result in substantial loss of material, resulting in an overall low yield.
Therefore, a need exists for methods of producing SiC crystalline material useful as silicon carbide seed crystals having reduced defects, in a cost effective and time efficient manner, to facilitate large scale production of semiconductor devices including such materials.